1. Field of the Invention
The present invention relates to an apparatus which controls the intensity of a laser beam emitted from a laser unit, and more particularly to a beam intensity-controlling apparatus adapted for use with a semiconductor laser unit incorporated in an image-forming apparatus, such as a laser printer or a digital copying machine.
2. Description of the Related Art
In recent years, an electrophotographic laser printer has been developed, which scans a photosensitive surface with a laser beam emitted from a laser unit for the purpose of exposure and which prints images and/or characters by utilization of an electrophotographic process. In this type of laser printer, the laser unit is a semiconductor laser unit. In general, the beam intensity of the laser beam emitted from the semiconductor laser unit has to be stabilized by a control device since the beam intensity is likely to vary in accordance with temperature.
There are various types of known control systems which can control the intensity of the laser beam emitted from the semiconductor laser unit. Of the known control systems, the control system which employs a general-purpose microcomputer or microprocessor incorporating an A/D converter is considered a low-cost type. In this type of control system, the semiconductor laser is energized to emit the laser beam and a detector detects the beam intensity of a laser beam during the interval between the image-recording operation performed with respect to a given page and the image-recording operation performed with respect to the succeeding page, and the detected beam intensity is supplied to the A/D converter, for reading. The beam intensity read by the A/D converter is compared with a reference signal by the microcomputer and its peripheral circuits, and the current used for the driving of the semiconductor laser unit is controlled on the basis of the comparison. In this manner, the intensity of the laser beam emitted from the semiconductor laser unit is stabilized.
According to the conventional control system mentioned above, the intensity of the laser beam emitted from the semiconductor laser unit is stabilized by causing the semiconductor laser unit to emit a laser beam in the interval which is between the image-recording operations performed with respect to two successive pages, i.e., in the interval in which the image-recording operation is not performed. If this control system is used in a certain type of image-forming apparatus (e.g., a laser printer or a digital copying machine to which a reversal development system is applied), the laser beam emitted in the interval between the image-recording operations may fall on a recording region. If this occurs, the image on the recording region is developed with toner and toner is used in vain.
According to another type of control system which may solve the above problem, the intensity of a laser beam emitted from the semiconductor laser unit is stabilized, with the semiconductor laser unit being energized in the region other than the recording region. According to this control system, however, the time during which the semiconductor laser unit is energized is comparatively short, and the A/D converter incorporated in the general-purpose microcomputer requires a long time for reliably reading the detected beam intensity of the semiconductor laser unit. This being so, it is necessary to employ a high-speed A/D converter, i.e., a costly A/D converter. It is also necessary to employ a high-speed microcomputer since high-speed processing is required from the reading of the detected beam intensity to the determination of the driving current to be supplied to the semiconductor laser unit.
In the conventional control apparatuses, the beam intensity of the light beam emitted from the semiconductor laser is controlled in a predetermined cycle. If the time required from the start of the beam intensity to the attainment of the target beam intensity is short, then the cycle of the image intensity control should be shortened, accordingly. In other words, the time intervals at which the image intensity is controlled should be shortened. It should also be noted that the beam intensity control continues in the same cycle even after the attainment of the target beam intensity. In the case where the beam intensity control is carried out simultaneously with the image formation control executed by the image formation-controlling microcomputer of the image-forming apparatus, the image information control has to be executed with the beam intensity control being carried out in the predetermined cycle. Since the processing speed of a general-purpose microcomputer is too slow for this purpose, it is necessary to employ a high-speed microcomputer, making the manufacturing cost of the control apparatus high.